Ligand field diagram for octahedral complex explain briefly

The ligand field diagram for octahedral complex is explained here.

In a free metal ion, all the five d-orbitals are degenerate (i.e, having equal energies). But according to crystal field theory, under the influence of ligand field, degenracy of d-orbital is destroyed and they split into two or more energy levels. The difference in energy between two sets of d-orbitals is called Crystal Field Splitting Energy (CFSE). The extent of splitting depends upon the strength of ligand. A strong ligand causes greater splitting whereas weak ligand causes smaller splitting.

In octahedral complex, under the influence of ligand field, degenerate five d-orbitals  split into two sets of orbitals -one is and another one is . orbital consists of and orbital consists of orbitals.

The crystal field splitting diagram for octahedral complex is attached here.

In the diagram, Δo is crystal field splitting energy (CFSE).

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the correct answer would be b) volume versus temperature, because they are also directly proportional.